1. Field of Invention
The present invention relates to a light emitting device. More particularly, the present invention relates to a structure of a light emitting diode.
2. Description of Related Art
The light emitting diode (LED), in comparing with the conventional light bulb, has significantly advantages, such as small volume, long lifetime, low voltage/current driving, non-brittle property, having no thermal issue when emitting light, containing no Hg in considering issue of environmental contamination, light emitting efficiency in saving power, and so on. In addition, the light emitting efficiency of the LED is continuously increasing in recent years, and therefore the LED has gradually taken the place of light bulb or fluorescent lamp in some application field, such as the scanner lamp with high responding speed, the back light used in liquid crystal display, the control panel light in a car, the traffic light, or the usual illuminating apparatus.
In addition, since the III–V nitride compound is a material with a broad band gap. The emitted wavelength can cover from ultraviolet light to infrared light. In other words, it has covered the whole range of visible light. Therefore, the light emitting device using the III–V nitride compound semiconductor, such as GaN, GaAlN, or GaInN, has been widely applied in various light modules.
FIG. 1 is a cross-sectional view, schematically illustrating the conventional LED structure. In FIG. 1, the LED structure is basically formed from a substrate 110, an n-type doping semiconductor layer 120, an electrode 122, a light emitting layer 130, a p-type doping semiconductor layer 140, an ohmic contact layer 150, and another electrode 142. The n-type doping semiconductor layer 120, the light emitting layer 130, the p-type doping semiconductor layer 140, the ohmic contact layer 150, and the electrode 142 are sequentially formed over the substrate 110, and the light emitting layer 130 just cover a portion of the n-type doping semiconductor layer 120. The electrode 122 is disposed on the n-type doping semiconductor layer 120 at the portion not being covered by the light emitting layer 130.
Still referring to FIG. 1, when electrons from the n-type doping semiconductor layer 120 recombine with the electron-holes from the p-type doping semiconductor layer 140 at the light emitting layer 130, then a light 102 can be produced. A portion of the light 102 can transmit through the ohmic contact layer 150 and the substrate 110, and respectively emit out from the top and the bottom of the LED structure 100. In addition, another portion of the light 102 is reflected by the surface of the substrate 110 or several interfacing surfaces between the electrode 142 and the substrate 110. For example, the light transversely propagates in the n-type doping semiconductor layer 120 between the substrate 110 and the light emitting layer 130. In this situation, a portion of optical energy of the light 102 is absorbed by the n-type doping semiconductor layer 120, the p-type doping semiconductor layer 140, the electrode 122, and the electrode 142, resulting in a lower external quantum efficiency for the LED structure 100.
In order to solve the foregoing issue, the disclosure in JP 11-274568 uses the chemical mechanical polishing process and etching process to randomly roughen the substrate surface of the LED structure, so as to allow the incident light on the substrate is scattered and therefore increases the external quantum efficiency of the LED structure.
However, the way to randomly roughen the surface of the substrate does not effectively increase the external quantum efficiency of the LED structure. In one hand, that is because if the recessing pattern or the protruding pattern on the substrate surface is over large, then the crystal quality of the n-type doping semiconductor layer 120 growing from the substrate surface is reduced. As a result, the internal quantum efficiency of the LED structure is reduced, and therefore the external quantum efficiency cannot be increased. In another hand, the substrate surface being randomly roughened causes the optical energy in transverse propagation to be more easily absorbed by this roughened surface. It then causes the decay of light, emitted from the LED structure, and the external quantum efficiency cannot be sufficiently obtained.